An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic dispersion composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates.
An electrophoretic dispersion may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the pigment particles to migrate to the opposite plate, thereby reversing the color.
Alternatively, an electrophoretic dispersion may have two types of pigment particles of contrasting colors and carrying opposite charge polarities and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to opposite ends. Thus one of the colors of the two types of the pigment particles would be seen at the viewing side.
Further alternatively, an electrophoretic dispersion may comprise more than two types of pigment particles, and therefore it is capable of displaying multiple color states.
For all types of the electrophoretic displays, the dispersion is undoubtedly one of the most crucial parts of the device. The composition of the dispersion determines, to a large extent, the lifetime, contrast ratio, switching rate and bistability of the device.
For the pigment particles in the dispersion, a polymer layer is usually grafted over their surface to facilitate dispersion of the pigment particles in the dispersing solvent and hence the polymer layer is generally solvent compatible. For example, when a hydrocarbon solvent is used as the dispersing solvent, it is desirable to select a polymer with long alkyl side chains as the outer coating layer over the pigment particles. Such surface modified pigment particles, however, cannot provide both good contrast ratio and image bistability, due to the strong inter-particle repulsion force introduced by the surface-grafted polymers.